Urediniospores were collected separately from rust pustules on
oregano (Origanum vulgare) and spearmint (Mentha spicata) plants purchased
from retail stores in Gainesville, Alachua Co., FL, in April 2004 and March
2005. Morphological characteristics of the urediniospores from both hosts were
consistent with descriptions of Puccinia menthae. Spearmint,
peppermint (M. x piperita), oregano (O. vulgare), sweet
marjoram (O. majorana), and Greek oregano (Oregano
heracleoticum) were inoculated with urediniospore suspensions of each rust
isolate in a series of cross-inoculation experiments. Rust developed on oregano,
sweet marjoram, and Greek oregano only when inoculated with urediniospores from
the oregano plant, and this rust isolate did not infect any of the Mentha
spp. tested. Rust developed on spearmint when inoculated with urediniospores
from the mint plant, but this isolate did not infect peppermint, oregano, sweet
marjoram, or Greek oregano. The two isolates of Puccinia menthae
appear to represent different populations, “spearmint rust” and “oregano rust.”
This is the first report of P. menthae on oregano in Florida.

Introduction

This report documents for the first time Puccinia menthae
Pers.:Pers. on oregano (Origanum vulgare L.) in Florida, although
P. menthae has been previously listed on Mentha spicata
L. (spearmint), M. aquatica (water mint), Mentha sp., and
Monarda punctata (horsemint) in Florida (2). Koike et al. (10)
previously reported P. menthae on oregano (O. vulgare)
and sweet marjoram (O. majorana L.) in California.
Puccinia menthae also has been reported on O. vulgare in other
countries, including Bulgaria, Canary Islands, China, Denmark, Germany,
Greece, Himalaya, India, Norway, Pakistan, Sweden, and United Kingdom (5).
Although summer temperatures may limit rust development in Florida, mint and
oregano plants are most commonly available for purchase in the cooler fall,
winter, and spring seasons. Rust on oregano and mint plants in commercial garden
centers has the potential to reduce product quality if conditions are favorable
for disease development.

The rust fungus was first encountered on oregano plants in Florida in April
2004, when leaves with rust pustules were collected from potted plants at a
residence in Gainesville, Alachua Co., FL. The sample was collected by a
graduate student, Matthew J. Moyer (Environmental Horticulture Department, University of
Florida IFAS) and submitted as a collection for the Fungal Plant Pathogens
course taught in the Plant Pathology Department, University of Florida IFAS, by the first author (C. M.
Stiles). Disease symptoms included small, circular to irregular, brown, necrotic
spots, some of which included uredinia (Fig. 1). Uredinia in the symptomatic
tissue on oregano were few in number, but a mint plant (Mentha sp.)
growing in the same pot also had developed similar rust symptoms with more
numerous uredinia. Attempts to re-inoculate new growth on the same plants with
urediniospores were unsuccessful, probably due to high greenhouse temperatures
(6,7) in summer 2004.

Fig. 1. A lesion and uredinia on a leaf of inoculated oregano.

In early March 2005, an oregano plant (O. vulgare) with rust symptoms
and pustules was purchased from a local retail store in Gainesville, Alachua
Co., FL, by C. M. Stiles. This plant originated from a Florida-based commercial
greenhouse operation. Symptoms and pustules were similar to those on the oregano
collected one year earlier. About a week later, additional, younger oregano
plants at the same store appeared to be free of rust; however, a mint plant (Mentha
sp.) with orange-brown rust uredinia (Fig. 2) was found at the store. On mint
plants, there was less necrotic tissue compared to oregano, however, uredinia
were more numerous, and there was some chlorosis associated with uredinia.
Urediniospores (Fig. 3) were similar in size and appearance on both the oregano
and mint plant. The mint plant was maintained in a lighted chamber at 21°C and
numerous pustules formed on upper leaves over the next two to three weeks.
However, in June 2005, mint plants observed at the same retail store appeared to
be free of rust, thus, high summer temperatures may limit rust development in
Florida as noted for peppermint rust (7) in other areas. Fletcher (6) found that
urediniospores of P. menthae failed to germinate at 25°C, and
germination was reduced at 20°C.

Fig. 2. Lesions and uredinia of Puccinia menthae on mint collected in March 2005.

Fig. 3. Urediniospores from pustules on inoculated oregano.

Ellipsoidal urediniospores (Fig. 3) from both oregano and mint had up to
three germ pores, consistent with descriptions of Puccinia menthae
Pers.:Pers. (1,10). Urediniospores from mint and oregano plants were measured
separately from plants collected in 2004 and 2005. Urediniospores from mint
plants ranged from 20 to 30 µm by 15 to 28 µm (mean = 25 by 22 µm, n = 31);
urediniospores from oregano plants ranged from 21 to 25 µm by 18 to 22 µm (mean = 24 by 20 µm, n = 19). These size ranges of urediniospores were also consistent
with descriptions of P. menthae (1,10). No teliospores were
observed on either host.

Cross-Inoculation Experiments with Oregano and Mint Rust Isolates

Urediniospores were collected from pustules on the oregano plant using a
vacuum-operated spore-collecting tool. Spore suspensions were prepared (~ 1.5 x 105 spores / ml water) using Tween 20 (1 drop / 300 ml suspension) to
disperse spores. Using a Crown Spra-tool propellent sprayer (Crown Industrial
Products Co., Hebron, IL), the inoculum suspension was sprayed onto leaves
(upper and lower surfaces) of spearmint (M. spicata), peppermint (M.
× piperita), and oregano (O. vulgare). Plants were placed in a dew
chamber at 18°C for about 24 h, then removed to a lighted growth chamber
(15 h fluorescent light) at 21°C. Each plant was covered with a plastic bag
for another 24 h to maintain humidity. Plants were then maintained in the
lighted chamber at 21°C and observed for development of symptoms and uredinia.
No rust developed on the spearmint or peppermint plants that were inoculated
with urediniospores from the oregano plant. However, after about two weeks, on
the oregano, small chlorotic and necrotic spots and rust pustules formed on both
leaves and stems (Fig. 4). Urediniospores were similar in size and shape to the
original collection.

Fig. 4. A lesion and uredinia on a stem of inoculated oregano.

To determine the host range of these isolates, urediniospores were collected
from the newly formed pustules on the inoculated oregano plant as well as from
the spearmint plant that was purchased from the retail store. Separate spore
suspensions from the two plant species (isolates hereafter termed “oregano rust”
and “spearmint rust”) were made and inoculated as described above. Due to the
limited quantity of initial inoculum, cross-inoculations were completed in a
series of four experiments onto oregano, Greek oregano (O. heracleoticum
L.), sweet marjoram, peppermint (M. × piperita), and spearmint
plants. One or two pots, with multiple plants and stems, of oregano, Greek
oregano, and sweet marjoram were inoculated with each isolate in three of the
four experiments, while peppermint and spearmint were included in all four
experiments. In all experiments, inoculated and non-inoculated plants were
maintained in the same incubator at 21°C. No rust pustules were observed on the
non-inoculated plants when plants were observed two to five weeks after
inoculation.

Inoculation with spores from oregano again resulted in necrotic lesions with
numerous pustules on oregano. On sweet marjoram inoculated with the oregano
isolate, numerous pustules formed; however there was less necrosis. Instead, a
chlorotic halo was sometimes observed around uredinia (Fig. 5), also observed by
Koike et al. (10). Infection also occurred on Greek oregano; however in two of
the three experiments, fewer lesions formed on leaves and fewer pustules formed
in the lesions, thus, disease development appeared to be less severe or delayed
on this species (Table 1). In one of the three experiments, only three pustules
were found on a total of 111 lesions observed on Greek oregano three weeks after
inoculation. In another experiment, only one of eight stems of one plant had a
few lesions on leaves three weeks after inoculation, while oregano plants
exhibited abundant lesions and pustules formed on leaves of all ages and on all
stems of plants. This suggests the oregano rust isolate may have a longer latent
period on Greek oregano, as has been reported when mint rust isolates are
inoculated on some related mint hosts (8).

Table 1. Pathogenicity of oregano and mint rust isolates on
various oregano, marjoram, and mint species. Urediniospores from oregano and
spearmint were collected separately and inoculated onto each of the plant hosts
listed in a series of cross-pathogenicity tests. Number of + signs indicate
relative abundance of lesions and pustules on the various hosts; “–” indicates host was
not infected.

Plant host

Oreganorust isolate

Spearmintrust isolate

oregano

Origanum vulgare

+ +

–

sweet marjoram

Origanum majorana

+ +

–

Greek oregano

Oregano heracleoticum

+*

–

peppermint

Mentha x piperata

–

–

spearmint

Mentha spicata

–

+ +

* On Greek oregano, fewer lesions formed on leaves and fewer
pustules formed in the lesions, in two of three experiments, compared to oregano
and sweet marjoram.

Inoculation with spores from the mint plant resulted in rust pustules only on
spearmint, but not on peppermint (Fig. 6), oregano, Greek oregano, or sweet
marjoram. When spores from the oregano rust isolate were inoculated onto
peppermint and spearmint, neither mint species developed symptoms or formed rust
pustules. Results of these inoculation experiments are summarized in Table 1.

Fig. 6A. Upper leaf surfaces of a spearmint leaf (left) and a peppermint leaf (right) inoculated with urediniospores of the “spearmint rust” isolate.

Cross-inoculation experiments in this study indicate there is host
specialization of P. menthae in Florida. The rust isolate from oregano
did infect oregano, Greek oregano, and sweet marjoram. This finding is
consistent with the report by Koike et al. (10), who found that California
isolates of P. menthae from oregano and sweet marjoram infected other
types of oregano and marjoram, but did not infect spearmint. However, the
oregano isolate in these studies also did not infect any of the other mint
species tested.

The rust isolate from spearmint did not infect oregano or the other related
plant species tested, which is consistent with an earlier study by Fletcher (6).
The spearmint isolate also did not infect peppermint, which is consistent with
studies of mint rust in the midwestern U.S. (3), the Pacific Northwest (8), and
in Australia (4). These other studies indicate there are two populations of mint
rust, indicated as ‘spearmint rust’ and ‘peppermint rust’ (4,8). Scanning
electron studies of urediniospores (9) indicate morphological differences
between the two populations of mint rusts. Morphological and molecular studies
of Australian isolates (4) suggest that the spearmint rust and peppermint rust
isolates could be taxonomically separated at the varietal or species level. Of
the previous studies, only Fletcher (6) attempted to inoculate oregano
with isolates of rust from spearmint; in that study, none of the eight isolates
produced a disease response in oregano. In the current studies, the rust isolate
from oregano did not infect any of the mint species tested, thus the oregano
rust population may be separate from either the spearmint rust or peppermint
rust. Further collections and inoculation experiments would be needed to
determine the complete host range of P. menthae in oregano and mint
species in Florida, and to determine whether races exist within populations of
P. menthae, as reported in the U.S. Pacific Northwest (8), the midwestern
U.S. (3), and other locations.

Literature Cited

1. Arthur, J. C. 1934. Manual of the rusts in United States and Canada.
Purdue Research Foundation, Lafayette, IN.